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Abstract:

An object of the present invention is to provide a golf ball intermediate
layer resin composition with an excellent resilience and fluidity.
Another object of the present invention is to provide a golf ball
intermediate layer resin composition with a high hardness and durability.
Yet another object of the present invention is to provide a golf ball
traveling a great distance with an ionomer intermediate layer. The
present invention provides a golf ball intermediate layer resin
composition comprising, (A) an ionomer resin consisting of a metal
ion-neutralized product of a binary copolymer composed of an olefin and
an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;
(B) a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and (C) a zinc compound, wherein a content ratio ((A)/(B)) of (A)
component to (B) component ranges from 50/50 to 80/20 in a mass ratio and
a content of (C) component ranges from 0.1 part to 20 parts with respect
to 100 parts by mass of a sum of (A) component and (B) component, and the
golf ball intermediate layer resin composition has a melt flow rate
(190° C., 2.16 kg) of 15 g/10 min or more.

Claims:

1. A golf ball intermediate layer resin composition comprising, (A) an
ionomer resin consisting of a metal ion-neutralized product of a binary
copolymer composed of an olefin and an α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms; (B) a binary copolymer
composed of an olefin and an α,β-unsaturated carboxylic acid
having 3 to 8 carbon atoms, and (C) a zinc compound, wherein a content
ratio ((A)/(B)) of (A) component to (B) component ranges from 50/50 to
80/20 in a mass ratio and a content of (C) component ranges from 0.1 part
to 20 parts with respect to 100 parts by mass of a sum of (A) component
and (B) component, and the golf ball intermediate layer resin composition
has a melt flow rate (190.degree. C., 2.16 kg) of 15 g/10 min or more.

2. The golf ball intermediate layer resin composition according to claim
1, wherein (A) the ionomer resin has the α,β-unsaturated
carboxylic acid component in a content of 15 mass % or more.

7. The golf ball intermediate layer resin composition according to claim
1, wherein A1, A2, B1, and B2 defined below satisfy following
expressions. P1=A1/(A1+B1) P2=A2/(A2+B2) 0.2.ltoreq.P1.ltoreq.1.0
0.3.ltoreq.P2.ltoreq.1.0 1.5.ltoreq.P2/P1.ltoreq.2.5 (A1 is defined as
an area under a peak around 1600 cm-1 and B1 is defined as an area
under a peak around 1700 cm-1 in a spectrum obtained by analyzing
the golf ball intermediate layer resin composition before injection
molding by FT-IR, and A2 is defined as an area under a peak around 1600
cm-1 and B2 is defined as an area under a peak around 1700 cm-1
in a spectrum obtained by analyzing the golf ball intermediate layer
composition after injection molding by FT-IR)

8. The golf ball intermediate layer resin composition according to claim
1, wherein a degree of neutralization of the carboxyl groups contained in
(A) the ionomer resin ranges from 15 mole% to 90 mole %.

13. A golf ball comprising a core having a center and at least one
intermediate layer disposed around the center and a cover disposed around
the core, wherein at least one of the intermediate layer is formed by
injection molding a golf ball intermediate layer resin composition which
comprises (A) an ionomer resin consisting of a metal ion-neutralized
product of a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;
(B) a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and (C) a zinc compound, wherein a content ratio ((A)/(B)) of (A)
component to (B) component ranges from 50/50 to 80/20 in a mass ratio and
a content of (C) component ranges from 0.1 part to 20 parts with respect
to 100 parts by mass of a sum of (A) component and (B) component, and the
golf ball intermediate layer resin composition has a melt flow rate
(190.degree. C., 2.16 kg) of 15 g/10 min or more.

14. The golf ball according to claim 13, wherein the intermediate layer
has a thickness ranging from 0.5 mm to 1.5 mm.

15. The golf ball according to claim 13, wherein the golf ball has a
cover with a thickness ranging from 0.1 mm to 2.0 mm.

16. The golf ball according to claim 13, wherein the intermediate layer
has a hardness ranging from 55 to 75 in Shore D hardness.

17. The golf ball according to claim 13, wherein (A) the ionomer resin
has the α,β-unsaturated carboxylic acid component in a content
of 15 mass % or more, and is neutralized with zinc.

20. The golf ball according to claim 13, wherein A1, A2, B1, and B2
defined below satisfy following expressions. P1=A1/(A1+B1)
P2=A2/(A2+B2) 0.2.ltoreq.P1.ltoreq.1.0 0.3.ltoreq.P2.ltoreq.1.0
1.5.ltoreq.P2/P1.ltoreq.2.5 (A1 is defined as an area under a peak around
1600 cm-1 and B1 is defined as an area under a peak around 1700
cm-1 in a spectrum obtained by analyzing the golf ball intermediate
layer resin composition before injection molding by FT-IR, and A2 is
defined as an area under a peak around 1600 cm-1 and B2 is defined
as an area under a peak around 1700 cm-1 in a spectrum obtained by
analyzing the golf ball intermediate layer resin composition after
injection molding by FT-IR).

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a golf ball intermediate layer
resin composition and a golf ball, in particular, an improvement of a
resilience and fluidity of an ionomer resin composition.

DESCRIPTION OF THE RELATED ART

[0002] Ionomer resins and polyurethane are used as materials for
constituting golf balls. Use of the ionomer resins as the constituting
material of the golf ball provides the golf ball traveling a great
distance, because of its high stiffness. Accordingly, ionomer resins are
widely used as a material constituting a cover or an intermediate layer
of the golf ball. However, there still remains a room for further
improvement with respect to the stiffness and fluidity thereof and
various improvements have been proposed for improving these properties.

[0003] Japanese Patent Publication No. 2000-157646 A discloses a golf ball
cover composition having a melt index (MI) of 1 dg/sec. or more and
primarily comprising a mixture of a base resin, blended with (d) a metal
soap obtained by neutralizing an organic acid having up to 29 carbon
atoms with a monovalent to trivalent metal ion, in a mass ratio of the
base resin to the metal soap being 95:5 to 80:20, wherein the base resin
comprises an ionomer resin component containing (a) a ternary ionomer
resin consisting of a metal ion neutralized product of an
olefin-unsaturated carboxylic acid-unsaturated carboxylate copolymer
having an acid content of 12 wt % or less, and (b) a binary ionomer resin
consisting of a metal ion neutralized product of an olefin-unsaturated
carboxylic acid copolymer having an acid content of 15 wt % or less in a
ratio of 40:60 to 100:0; and (c) an unneutralized random copolymer
composed of olefin and unsaturated carboxylic acid monomers, in a mass
ratio of the ionomer resin component to (c) the unneutralized random
copolymer being 75:25 to 100:0.

[0004] U.S. Pat. No.5,306,760 discloses a golf ball comprising a core and
a cover, wherein the cover consists essentially of 100 parts by weight of
at least one ionomer resin and from about 25 to about 100 parts by weight
of fatty acid salt, wherein said ionomer resin is the reaction product of
an olefin having from 2 to 8 carbon atoms and an unsaturated
monocarboxylic acid having from 3 to 8 carbon atoms.

[0005] U.S. Pat. No.5,312,857 discloses a golf ball comprising a core and
a cover, wherein the cover consists essentially of 100 parts by weight of
at least one ionomer resin and from about 25 to about 100 parts by weight
of a metal stearate, wherein said ionomer resin is the reaction product
of an olefin having from 2 to 8 carbon atoms and an unsaturated
monocarboxylic acid having from 3 to 8 carbon atoms.

[0006] Japanese Patent Publication No. H06-292740 A discloses a
composition for a golf ball cover material which comprises (A) 15 to 90
parts by weight of metal salts of an ethylene/ethylenically unsaturated
monocarboxylic acid copolymer having an ethylenically unsaturated
monocarboxylic acid content of 10 to 30% by weight and a degree of
neutralization of at least of 25 mole % and (B) 85 to 10 parts by weight
of an ethylene/(meth)acrylate ester/ethylenically unsaturated
monocarboxylic acid terpolymer having a (meth)acrylate ester content of
12 to 45% by weight and an ethylenically unsaturated monocarboxylic acid
content of 0.5 to 5% by weight.

[0007] Japanese Patent Publication No. 2001-218873 A discloses a
multi-piece golf ball comprising a solid core, an intermediate layer
enclosing the solid core, and a cover enclosing the intermediate layer,
wherein at least one of said intermediate layer and said cover is formed
of a heated mixture comprising (a) 100 parts by weight of an
olefin-unsaturated carboxylic acid random copolymer or an
olefin-unsaturated carboxylic acid-unsaturated carboxylate random
copolymer or both of them, (b) 5 to 80 parts by weight of a fatty acid
having a molecular weight of at least 280 or a derivative thereof, and
(c) 0.1 to 10 parts by weight of a basic inorganic metal compound capable
of neutralizing acid groups in components (a) and (b), said heated
mixture having a melt index of at least 1.0 dg/min, and wherein said
intermediate layer has a Shore D hardness of 40 to 63, said cover has a
Shore D hardness of 45 to 68, and the Shore D hardness of said solid core
at its center is not greater than the Shore D hardness of said
intermediate layer, which is not greater than the Shore D hardness of
said cover.

[0008] Japanese Patent Publication No. 2002-219195 A discloses a golf ball
material comprising a mixture which is composed of essential components:
100 pars by weight of a resinous component consisting of a base resin and
(e) a non-ionomer thermoplastic elastomer, the base resin and the
elastomer being blended in a weight ratio of 100:0 to 50:50; (c) 5 to 80
parts by weight of a fatty acid and/or fatty acid derivative having a
molecular weight of 280 to 1,500; and (d) 0.1 to 10 parts by weight of a
basic inorganic metal compound capable of neutralizing acidic groups left
unneutralized in the base resin and component (c), wherein the base resin
has (a) an olefin-unsaturated carboxylic acid binary random copolymer
and/or a metal ion-neutralized olefin-unsaturated carboxylic acid binary
random copolymer, blended with (b) an olefin-unsaturated carboxylic
acid-unsaturated carboxylate ternary random copolymer and/or a metal ion
neutralized olefin-unsaturated carboxylic acid-unsaturated carboxylate
ternary random copolymer, in a weight ratio of 100:0 to 25:75.

SUMMARY OF THE INVENTION

[0009] The improvement in a flight distance of a golf ball using an
ionomer resin as an intermediate layer material has been addressed. An
approach to improve a flight distance is to enlarge a diameter of a
center part having a high resilience. In order to enlarge a diameter of
the center, it is necessary to mold a thinner intermediate layer.
However, it is difficult to mold a thin intermediate layer by an
injection molding method. In addition, the thin intermediate layer often
causes a lower durability. Another approach to improve a flight distance
is to use an ionomer resin having a high degree of neutralization.
However, since the ionomer resin having a high degree of neutralization
has low fluidity, it is extremely difficult to injection mold a thin
intermediate layer. As a method of improving the fluidity of the ionomer
resin, it is known that a low molecular weight material such as a fatty
acid or a metal salt thereof is added to the ionomer resin having a high
degree of neutralization. However, since a considerable amount of the low
molecular weight material must be added in order to improve the fluidity,
the low molecular weight material tends to bleed out from the surface of
the intermediate layer, which causes a problem of lower adhesion of the
intermediate layer to the cover. As another method of improving the
fluidity of the ionomer resin, it is known that an ethylene-(meth)acrylic
acid binary copolymer or an ethylene-(meth)acrylic acid-(meth)acrylic
acid ester ternary copolymer is added to the ionomer resin. However, the
hardness of the resultant blend becomes low, which causes a shorter
flight distance because of an increased spin rate on driver shots.

[0010] As described above, in order to improve a flight distance of the
golf ball using an ionomer resin as an intermediate layer material, it is
necessary to use an intermediate material having a high resilience, mold
a thin intermediate layer, and increase an intermediate layer hardness.
However, since the ionomer resin having a high degree of neutralization
has a low fluidity, it is difficult to mold a thin intermediate layer.
Further, the intermediate layer having a high hardness causes a problem
of lower durability.

[0011] The present invention has been achieved in view of the above
circumstances. An object of the present invention is to provide a golf
ball intermediate layer resin composition with an excellent resilience
and fluidity. Another object of the present invention is to provide a
golf ball intermediate layer resin composition with a high hardness and
durability. Yet another object of the present invention is to provide a
golf ball traveling a great distance with an ionomer intermediate layer.

[0012] The present invention provides a golf ball intermediate layer resin
composition comprising, (A) an ionomer resin consisting of a metal
ion-neutralized product of a binary copolymer composed of an olefin and
an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;
(B) a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and (C) a zinc compound, wherein a content ratio ((A)/(B)) of (A)
component to (B) component ranges from 50/50 to 80/20 in a mass ratio and
a content of (C) component ranges from 0.1 part to 20 parts with respect
to 100 parts by mass of a sum of (A) component and (B) component, and the
golf ball intermediate layer resin composition has a melt flow rate
(190° C., 2.16 kg) of 15 g/10 min or more.

[0013] (A) The binary ionomer resin mainly contained in the golf ball
intermediate layer resin composition of the present invention improves
the resilience of the resultant golf ball. Further, (B) the binary
copolymer composed of an olefin and an α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms improves the fluidity of the
golf ball intermediate layer resin composition. Further, the golf ball
intermediate layer resin composition of the present invention features
(C) a zinc compound. (C) The zinc compound is used to neutralize
non-neutralized carboxyl groups existing in (A) component and (B)
component, thereby improving a resilience of the resin composition. Since
(C) the zinc compound with a relatively slow neutralization rate is used,
the neutralization does not proceed so much in a melt-mixing step or an
extruding step of the golf ball intermediate layer resin composition.
Thus, the golf ball intermediate layer resin composition maintains a good
fluidity. Since the neutralization proceed in a step of molding in a
mold, the resultant golf ball intermediate layer resin composition has as
high resilience as the ionomer resin having a high neutralization degree.
Consequently, the golf ball intermediate layer resin composition of the
present invention enables to strike a balance between the fluidity and
resilience. Further, according to the present invention, use of (C) the
zinc compound for neutralizing non-neutralized carboxyl groups existing
in (A) component and (B) component provides a golf ball having a good
durability, irrespective of the resultant intermediate layer having a
high hardness. Yet further, since the golf ball intermediate layer resin
composition of the present invention does not contain a low molecular
material such as a fatty acid or a metal salt thereof which bleeds out
from the surface of the core, the adhesion to the cover is excellent.

[0014] The present invention includes the golf ball intermediate layer
resin composition before carboxyl groups existing in (A) component and
(B) component are neutralized with (C) the zinc compound, and the golf
ball intermediate layer resin composition after the neutralization
proceeded. It is noted that the golf ball intermediate layer resin
composition after the neutralization proceeded includes the composition
after the neutralization proceeded partially. In the present invention,
the golf ball intermediate layer resin composition before neutralization
may be referred to as "non-neutralized golf ball intermediate layer resin
composition," and the golf ball intermediate layer resin composition
after neutralization may be referred to as "neutralized golf ball
intermediate layer resin composition," respectively. Simply referred
"golf ball intermediate layer resin composition" includes both
embodiments, unless otherwise described.

[0015] The present invention further provides a golf ball comprising a
core having a center and at least one intermediate layer disposed around
the core, and a cover disposed over the core, wherein at least one of the
intermediate layer is formed by injection molding the golf ball
intermediate layer resin composition of the present invention. The
intermediate layer preferably has a thickness ranging from 0.5 mm to 1.5
mm.

[0016] According to the present invention, the golf ball intermediate
layer resin composition with an excellent resilience and fluidity is
obtained. Further, the golf ball intermediate layer resin composition of
the present invention has a high hardness and durability. According to
the present invention, the golf ball traveling a great distance with an
ionomer intermediate layer is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a FT-IR spectrum of one embodiment of the golf ball
intermediate layer resin composition of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] The present invention provides a golf ball intermediate layer resin
composition comprising, (A) an ionomer resin consisting of a metal
ion-neutralized product of a binary copolymer composed of an olefin and
an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;
(B) a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and (C) a zinc compound, wherein a content ratio ((A)/(B)) of (A)
component to (B) component ranges from 50/50 to 80/20 in a mass ratio and
a content of (C) component ranges from 0.1 part to 20 parts with respect
to 100 parts by mass of a sum of (A) component and (B) component, and the
golf ball intermediate layer resin component has a melt flow rate
(190° C., 2.16 kg) of 15 g/10 min or more.

[0019] First, (A) the ionomer resin consisting of a metal ion-neutralized
product of a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms
(hereinafter, sometimes merely referred to as "binary ionomer resin")
will be explained. (A) The ionomer resin includes, for example, one
prepared by neutralizing at least a part of carboxyl groups in a binary
copolymer composed of an olefin and an α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms with a metal ion. The olefin
preferably includes an olefin having 2 to 8 carbon atoms. Examples of the
olefin are ethylene, propylene, butene, pentene, hexene, heptene, and
octene. The olefin more preferably includes ethylene. Examples of the
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms are
acrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonic
acid. Among these, acrylic acid and methacrylic acid are particularly
preferred. As (A) the binary ionomer resin, preferred is the metal
ion-neutralized product of the binary copolymer composed of
ethylene-(meth)acrylic acid. Herein, "(meth)acrylic acid" means acrylic
acid and/or methacrylic acid.

[0020] The content of the α,β-unsaturated carboxylic acid
having 3 to 8 carbon atoms in (A) the binary ionomer resin is preferably
15 mass % or more, more preferably 16 mass % or more, even more
preferably 17 mass % or more, and is preferably 30 mass % or less, more
preferably 25 mass % or less. If the content of the
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is
15 mass % or more, the resultant intermediate layer has a desirable
hardness. If the content of the α,β-unsaturated carboxylic
acid having 3 to 8 carbon atoms is 30 mass % or less, since the hardness
of the resultant intermediate layer does not become excessively high, the
durability and shot feeling become better.

[0021] The degree of neutralization of the carboxyl groups contained in
the binary ionomer resin is preferably 15 mole % or more, more preferably
20 mole % or more, and is preferably 90 mole % or less, more preferably
85 mole % or less. If the degree of neutralization is 15 mole % or more,
the resultant golf ball has better resilience and durability. If the
degree of neutralization is 90 mole % or less, the golf ball intermediate
layer resin composition has better fluidity (good moldability). The
degree of neutralization of the carboxyl groups in the binary ionomer
resin can be calculated by the following expression.

Degree of neutralization (mol %) of the binary ionomer resin=(the number
of moles of carboxyl groups neutralized in the binary ionomer resin/the
number of moles of all carboxyl groups contained in the binary ionomer
resin)×100

[0022] Examples of a metal (ion) used for neutralizing at least a part of
carboxyl groups of the binary ionomer resin include: monovalent metals
ions such as sodium, potassium, lithium, or the like; divalent metals
ions such as magnesium, calcium, zinc, barium, cadmium, or the like;
trivalent metals ions such as aluminum or the like; and other metals
(ions) such as tin, zirconium, or the like. (A) The binary ionomer resin
used in the present invention is preferably neutralized with zinc. Use of
(A) the binary ionomer resin neutralized with zinc provides a golf ball
with a good durability and low temperature durability.

[0026] The binary ionomer resins may be used alone or as a mixture of at
least two of them. It is noted that Na, Zn, Li, and Mg described in the
parentheses after the trade names indicate metal types of neutralizing
metal ions of the binary ionomer resins.

[0027] (A) The binary ionomer resin preferably has a bending stiffness of
140 MPa or more, more preferably 150 MPa or more, even more preferably
160 MPa or more, and preferably has a bending stiffness of 550 MPa or
less, more preferably 500 MPa or less, even more preferably 450 MPa or
less. If the bending stiffness of (A) the binary ionomer resin is too
low, the flight distance tends to be shorter because of the increased
spin rate on driver shots. If the bending stiffness is too high, the
durability of the golf ball may be lowered.

[0028] The melt flow rate (190° C., 2.16 kg) of (A) the binary
ionomer resin is preferably 0.1 g/10 min or more, more preferably 0.5
g/10 min or more, and even more preferably 1.0 g/10 min or more, and is
preferably 30 g/10 min or less, more preferably 20 g/10 min or less, even
more preferably 15 g/10 min or less. If the melt flow rate of (A) the
binary ionomer resin is 0.1 g/10 min or more, the golf ball intermediate
layer resin composition has better fluidity, and for example, it is easy
to mold the thin-walled intermediate layer. If the melt flow rate of (A)
the binary ionomer resin is 30 g/10 min or less, the durability of the
resultant golf ball becomes better.

[0029] (A) The binary ionomer resin preferably has a slab hardness of 50
or more, more preferably 55 or more, even more preferably 60 or more, and
preferably has a slab hardness of 75 or less, more preferably 73 or less,
even more preferably 70 or less in Shore D hardness. If the binary
ionomer resin has a slab hardness of 50 or more in Shore D hardness, the
resultant intermediate layer has a high hardness. If the binary ionomer
resin has a slab hardness of 75 or less in Shore D hardness, the
resultant intermediate layer does not become excessively hard and thus
the obtained golf ball has better durability.

[0030] Next, (B) the binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms
(hereinafter, sometimes merely referred to as "binary copolymer") will be
explained. The binary copolymer enhances the fluidity of the golf ball
intermediate layer resin composition.

[0031] (B) The binary copolymer is a nonionic copolymer of an olefin and
an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms
wherein the carboxyl groups thereof are not neutralized. Examples of the
olefin and the α,β-unsaturated carboxylic acid having 3 to 8
carbon atoms include the same olefin and α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms exemplified as constituents of
"(A) the binary ionomer resin." (B) The binary copolymer preferably
includes a binary copolymer composed of ethylene and (meth)acrylic acid.

[0032] The melt flow rate (190° C., 2.16 kg) of (B) the binary
copolymer is preferably 100 g/10 min or more, more preferably 150 g/10
min or more, and even more preferably 200 g/10 min or more, and is
preferably 1,500 g/10 min or less, more preferably 1,000 g/10 min or
less, even more preferably 800 g/10 min or less. If the melt flow rate
(190° C., 2.16 kg) of (B) the binary copolymer is 100 g/10 min or
more, the golf ball intermediate layer resin composition has better
fluidity, and thus it is easy to mold a thin intermediate layer. If the
melt flow rate (190° C., 2.16 kg) of (B) the binary copolymer is
1,500 g/10 min or less, the resultant golf ball has better durability.

[0034] Next, (C) the zinc compound will be explained. (C) The zinc
compound is used to neutralize non-neutralized carboxyl groups existing
in (A) component and (B) component, thereby improving the resilience of
the resin composition. Since (C) the zinc compound with a relatively slow
neutralization rate is used, the neutralization does not proceed so much
in a melt-mixing step or an extruding step of the golf ball intermediate
layer resin composition. Thus, the golf ball intermediate layer resin
composition maintains a good fluidity. Since the neutralization proceed
in a step of molding in a mold, the resultant golf ball intermediate
layer resin composition has as high a resilience as the ionomer resin
having a high neutralization degree. Examples of (C) the zinc compounds
are an oxide (zinc oxide), a hydroxide (zinc hydroxide), a sulfide (zinc
sulfide), a phosphide, a halide (zinc fluoride, zinc chloride, zinc
bromide, zinc iodide), a salt of an oxoacid (zinc sulfate, zinc
carbonate, zinc phosphate), an organic zinc compound (dimethylzinc,
diphenylzinc, etc), and a complex compound, in addition to a zinc powder.
Among them, as (C) the zinc compound, preferred is zinc oxide (ZnO), zinc
hydroxide (Zn(OH)2), or zinc carbonate (ZnCO3). Since these
zinc compound have an appropriate neutralizing ability, it is possible to
strike a balance between the fluidity and resilience. (C) The zinc
compound, without limitation, preferably has shapes such as a granular
form, plate-like form, needle-like form, and a tetra pod form (for
example, "Pana-tetra" available from AMTEC. Co., Ltd.). It is noted that
the zinc oxide is used as a pigment or a specific gravity adjusting agent
for the intermediate layer, but the surface of the zinc oxide for use in
the pigment or the specific gravity adjusting agent is often treated in
order to improve the dispersibility to the intermediate layer. On the
other hand, the zinc oxide used in the present invention is preferably
not surface-treated or slightly surface-treated in order to enhance the
neutralization ability.

[0035] If the golf ball intermediate layer resin composition before
injection molding is analyzed with FT-IR (Fourier transform infrared
spectrophotometer), A1 is defined as an area under a peak around 1600
cm-1 and B1 is defined as an area under a peak around 1700 cm-1
in the obtained spectrum and P1 is defined by the following equation;
P1=A1/(A1+B1). P1 is preferably 0.20 or more, more preferably 0.21 or
more, even more preferably 0.25 or more, and is preferably 1.0 or less,
more preferably 0.99 or less, even more preferably 0.5 or less. If P1
falls within the above range, the golf ball intermediate layer resin
composition has a good fluidity at the injection molding. In the
equation, A1 corresponds to an area under the peak attributed to the
neutralized carboxyl group, and B1 corresponds to an area under the peak
attributed to the non-neutralized carboxyl group. P1 is defined as a
ratio of the neutralized carboxyl groups to the whole carboxyl groups and
indicates a degree of neutralization of the golf ball intermediate layer
resin composition.

[0036] If the golf ball intermediate layer resin composition after
injection molding is analyzed with FT-IR (Fourier transform infrared
spectrophotometer), A2 is defined as an area under a peak around 1600
cm-1 and B2 is defined as an area under a peak around 1700 cm-1
in the obtained spectrum and P2 is defined by the following equation;
P2=A2/(A2+B2). P2 is preferably 0.3 or more, more preferably 0.31 or
more, and is preferably 1.0 or less, more preferably 0.99 or less. If P2
falls within the above range, the golf ball intermediate layer resin
composition has a good resilience. In the equation, A2 corresponds to an
area under the peak attributed to the neutralized carboxyl group, and B2
corresponds to an area under the peak attributed to the non-neutralized
carboxyl group. P2 is defined as a ratio of the neutralized carboxyl
groups to the whole carboxyl groups and indicates a degree of
neutralization of the golf ball intermediate layer resin composition.

[0037] P2/P1 is preferably 1.5 or more, more preferably 1.6 or more, even
more preferably 1.7 or more and is preferably 2.5 or less, more
preferably 2.4 or less, even more preferably 2.3 or less. P2/P1 indicates
a change in the degree of the neutralization of the golf ball before and
after the injection molding. If P2/P1 is equal to or more than the lower
limit, the effect of improving the resilience by the neutralization with
the zinc compound becomes large, while if P2/P1 is equal to or less than
the upper limit, the neutralization does not proceed excessively. Thus,
the golf ball intermediate layer resin composition maintains the fluidity
and the injection molding becomes easy.

[0038] In the golf ball intermediate layer resin composition, a content
ratio (A)/(B) of (A) the binary ionomer resin to (B) the binary copolymer
preferably ranges from 50/50 to 80/20, more preferably from 55/45 to
75/25, even more preferably 60/40 to 70/30 in a mass ratio. If the
content ratio (A)/(B) of (A) component to (B) component falls within the
above range, it becomes easy to strike a balance between the high
hardness and high fluidity.

[0039] The content of (C) the zinc compound contained in the golf ball
intermediate layer resin composition of the present invention is
preferably 0.1 part by mass or more, more preferably 0.2 part by mass or
more, even more preferably 0.5 part by mass or more, and is preferably 20
parts by mass or less, more preferably 15 parts by mass or less, even
more preferably 10 parts by mass or less with respect to 100 parts by
mass of a sum of (A) component and (B) component. If the content of (C)
the zinc compound is 0.1 part by mass or more with respect to 100 parts
by mass of a sum of (A) component and (B) component, the durability of
the obtained golf ball improves. On the other hand, if the content of (C)
the zinc compound is 20 parts by mass or less with respect to 100 parts
by mass of a sum of (A) component and (B) component, the golf ball
intermediate layer resin composition can maintain the fluidity.

[0040] In the present invention, the golf ball intermediate layer resin
composition may further contain a pigment component such as a white
pigment (for example, titanium oxide), a blue pigment or the like; a
specific gravity adjusting agent; a dispersant; an antioxidant; an
ultraviolet absorber; a light stabilizer; a fluorescent material; a
fluorescent brightener; or the like, as long as they do not impair the
effect of the present invention. In the golf ball intermediate layer
resin composition, a fatty acid or a metal salt thereof may be used in
combination as a fluidity improving agent to the extent that the effect
of the present invention does not deteriorate. However, it is not
preferable that the fatty acid or the metal salt thereof is used in
combination, because low molecular weight materials such as the fatty
acid and the metal salt thereof may cause low adhesion to the cover and
low mechanical properties of the intermediate layer.

[0041] The golf ball intermediate layer resin composition of the present
invention preferably consists of (A) the binary ionomer resin and (B) the
binary copolymer as a resin component, but may further contain another
thermoplastic elastomer or another thermoplastic resin to the extent that
the effect of the present invention does not deteriorate. In this case,
the total content of (A) the binary ionomer resin and (B) the binary
copolymer in the resin component of the golf ball intermediate layer
resin composition is preferably 50 mass % or more, more preferably 70
mass % or more, even more preferably 90 mass % or more.

[0042] Examples of other thermoplastic elastomers are a thermoplastic
polyamide elastomer having a commercial name of "Pebax (registered
trademark) (e.g. "Pebax 2533")" commercially available from Arkema K. K.;
a polyurethane elastomer having a commercial name of "Elastollan
(registered trademark) (e.g. "Elastollan XNY85A")" commercially available
from BASF Japan Ltd; a thermoplastic polyester elastomer having a
commercial name of "Hytrel (registered trademark) (e.g. "Hytrel 3548",
"Hytrel 4047")" commercially available from Du Pont-Toray Co.,Ltd.; a
thermoplastic polystyrene elastomer having a commercial name of "Rabalon
(registered trademark) (e.g. "Rabalon T3221C")" commercially available
from Mitsubishi Chemical Corporation. Examples of the thermoplastic resin
include an ionomer resin of the ternary copolymer composed of an olefin,
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and α,β-unsaturated carboxylic acid ester.

[0043] The golf ball intermediate layer resin composition of the present
invention can be obtained, for example, by dry blending (A) the binary
ionomer resin, (B) the binary copolymer, and (C) the zinc compound. The
dry blended mixture may be extruded in the form of pellet. The dry
blending may be carried out using for example, a mixer capable of
blending a raw material in the form of pellet, more preferably a tumbler
type mixer. Extruding can be carried out by publicly known extruders such
as a single-screw kneading extruder, a twin-screw kneading extruder, and
a twin-single kneading extruder. The extruding condition is not
particularly limited. For example, in the case of extruding with a
twin-screw kneading extruder, the preferable conditions are screw
diameter=45 mm; screw revolutions=50 rpm to 400 rpm; screw L/D=35, and
cylinder temperature; 140° C. to 200° C. If the extruding
temperature exceeds 200° C., the neutralization proceeds and the
fluidity may be lowered.

[0044] The melt flow rate (190° C., 2.16 kg) of the golf ball
intermediate layer resin composition of the present invention is
preferably 15 g/10 min or more, more preferably 16 g/10 min or more, and
even more preferably 18 g/10 min or more, and is preferably 100 g/10 min
or less, more preferably 70 g/10 min or less, even more preferably 40
g/10 min or less. If the melt flow rate of the golf ball intermediate
layer resin composition falls within the above range, the moldability
becomes better.

[0045] The golf ball intermediate layer resin composition of the present
invention preferably has a slab hardness of 55 or more, more preferably
57 or more, even more preferably 60 or more, and preferably has a slab
hardness of 75 or less, more preferably 73 or less, even more preferably
70 or less in Shore D hardness. Use of the golf ball intermediate layer
resin composition having a slab hardness of 55 or more in Shore D
hardness reduces the spin rate of the resultant golf ball on driver
shots. Thus, the golf ball traveling a great distance is obtained. On the
other hand, use of the golf ball intermediate layer resin composition
having a slab hardness of 75 or less in Shore D hardness provides a golf
ball with excellent durability. Herein, the slab hardness of the golf
ball intermediate layer resin composition is a measured hardness of the
golf ball intermediate layer resin composition that is molded into a
sheet form by a measuring method described later.

[0046] The golf ball intermediate layer resin composition of the present
invention preferably has a bending stiffness of 30 MPa or more, more
preferably 35 MPa or more, even more preferably 40 MPa or more, and
preferably has a bending stiffness of 350 MPa or less, more preferably
300 MPa or less, even more preferably 250 MPa or less. If the bending
stiffness of the golf ball intermediate layer resin composition is 30 MPa
or more, since the obtained golf ball has an outer-hard inner soft
structure, the flight distance becomes great. On the other hand, if the
bending stiffness of the golf ball intermediate layer resin composition
is 350 MPa or less, the obtained golf ball becomes appropriately soft and
thus the shot feeling becomes good.

[0047] The melt flow rate, bending stiffness, and the slab hardness of the
golf ball intermediate layer resin composition can be adjusted by
appropriately selecting kinds, amount or the like of (A) the binary
ionomer resin, (B) the binary copolymer, and (C) the zinc compound.

[0048] The golf ball of the present invention is not limited, as long as
the golf ball comprises a core having a center and at least one
intermediate layer disposed around the center and a cover disposed around
the core, wherein at least one of the intermediate layer is formed from
the above golf ball intermediate layer resin composition. For example, in
a two-piece golf ball comprising a single-layered core and a cover
disposed around the core, in a three-piece golf ball comprising a core
having a center and a single-layered intermediate layer disposed around
the center, and a cover disposed around the core, or in a multi-piece
golf ball comprising a core having a center and at least one intermediate
layer disposed around the center, and a cover disposed around the core
(including the three-piece golf ball mentioned above), at least one of
the intermediate layer is formed from the above golf ball intermediate
layer resin composition.

[0049] In the followings, the present invention will be explained based on
the preferable golf ball (including a three-piece golf ball) that
comprises a core having a center and at least one intermediate layer
disposed around the center and a cover disposed around the core, wherein
at least one of the intermediate layer is formed from the above golf ball
intermediate layer resin composition.

[0050] The core of the golf ball of the present invention preferably
includes, for example, a multi-layered core having a center and at least
one intermediate layer covering the center. The core preferably has a
spherical shape. If the core does not have a spherical shape, the cover
does not have a uniform thickness. As a result, there exist some portions
where the performance of the cover is lowered.

[0051] On the other hand, the center generally has the spherical shape,
but the center may be provided with a rib on the surface thereof so that
the surface of the spherical center is divided by the ribs. For example,
the surface of the spherical center is evenly divided by the ribs. In one
embodiment, the ribs are preferably formed on the surface of the
spherical center in an integrated manner, and in another embodiment, the
ribs are formed as an intermediate layer on the surface of the spherical
center. The ribs are preferably formed along an equatorial line and
meridians that evenly divide the surface of the spherical center, if the
spherical center is assumed as the earth. For example, if the surface of
the spherical center is evenly divided into 8, the ribs are formed along
the equatorial line, any meridian as a standard, and meridians at the
longitude 90 degrees east, longitude 90 degrees west, and the longitude
180 degrees east(west), assuming that the meridian as the standard is at
longitude 0 degree. If the ribs are formed, the depressed portion divided
by the ribs are preferably filled with a plurality of intermediate layers
or with a single-layered intermediate layer that fills each of the
depressed portions to make a core in the spherical shape. The shape of
the ribs, without limitation, includes an arc or an almost arc (for
example, a part of the arc is removed to obtain a flat surface at the
cross or orthogonal portions thereof).

[0052] A conventionally known rubber composition (hereinafter, sometimes
simply referred to as "center rubber composition") may be employed for
the center of the golf ball of the present invention, and the center can
be molded by, for example, heat-pressing a rubber composition containing
a base rubber, a crosslinking initiator, a co-crosslinking agent, and a
filler.

[0053] As the base rubber, a natural rubber and/or a synthetic rubber may
be used. Examples of the base rubber are a polybutadiene rubber, a
natural rubber, a polyisoprene rubber, a styrene polybutadiene rubber,
and ethylene-propylene-diene terpolymer (EPDM). Among them, typically
preferred is the high cis-polybutadiene having cis-1,4-bond in a
proportion of 40% or more, more preferably 70% or more, even more
preferably 90% or more in view of its superior resilience property.

[0054] The crosslinking initiator is blended to crosslink the base rubber
component. As the crosslinking initiator, an organic peroxide is
preferably used. Examples of the organic peroxide for use in the present
invention are dicumyl peroxide,
1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane,
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Among
them, dicumyl peroxide is preferable. An amount of the crosslinking
initiator to be blended in the rubber composition is preferably 0.3 part
by mass or more, more preferably 0.4 part by mass or more, and is
preferably 5 parts by mass or less, more preferably 3 parts by mass or
less based on 100 parts by mass of the base rubber. If the amount is less
than 0.3 part by mass, the center becomes too soft, and the resilience
tends to be lowered, and if the amount is more than 5 parts by mass, the
amount of the co-crosslinking agent must be increased in order to obtain
the appropriate hardness, which tends to cause the insufficient
resilience.

[0055] The co-crosslinking agent is not particularly limited, as long as
it serves to crosslink a rubber molecule by graft polymerization to a
base rubber molecular chain; for example, α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms or a metal salt thereof, more
preferably acrylic acid, methacrylic acid or a metal salt thereof may be
used. As the metal constituting the metal salt, for example, zinc,
magnesium, calcium, aluminum and sodium may be used, and among them, zinc
is preferred because it provides high resilience.

[0056] The amount of the co-crosslinking agent to be used is preferably 10
parts or more, more preferably 15 parts or more, even more preferably 20
parts or more, and is preferably 55 parts or less, more preferably 50
parts or less, even more preferably 48 parts or less, based on 100 parts
of the base rubber by mass. If the amount of the co-crosslinking agent to
be used is less than 10 parts by mass, the amount of the crosslinking
initiator must be increased to obtain an appropriate hardness, which
tends to lower the resilience. On the other hand, if the amount of the
co-crosslinking agent to be used is more than 55 parts by mass, the
center becomes too hard, so that the shot feeling may be lowered.

[0057] The filler contained in the center rubber composition is mainly
blended as a specific gravity adjusting agent in order to adjust the
specific gravity of the golf ball obtained as the final product, and may
be blended as required. Examples of the filler include an inorganic
filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium
oxide, tungsten powder, and molybdenum powder. The amount of the filler
to be blended in the rubber composition is preferably 0.5 part or more,
more preferably 1 part or more, and is preferably 30 parts or less, more
preferably 20 parts or less based on 100 parts of the base rubber by
mass. If the amount of the filler to be blended is less than 0.5 part by
mass, it becomes difficult to adjust the weight, while if it is more than
30 parts by mass, the weight ratio of the rubber component becomes small
and the resilience tends to be lowered.

[0058] As the center rubber composition, an organic sulfur compound, an
antioxidant or a peptizing agent may be blended appropriately in addition
to the base rubber, the crosslinking initiator, the co-crosslinking agent
and the filler.

[0059] As the organic sulfur compound, diphenyl disulfide or a derivative
thereof may be preferably used. Examples of the diphenyl disulfide or the
derivative thereof include diphenyl disulfide; mono-substituted diphenyl
disulfide such as bis(4-chlorophenyl)disulfide,
bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide,
bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide,
bis(4-iodophenyl)disulfide and bis(4-cyanophenyl)disulfide;
di-substituted diphenyl disulfide such as
bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,
bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,
bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide,
and bis(2-cyano-5-bromophenyl)disulfide; tri-substituted diphenyl
disulfide such as bis(2,4,6-trichlorophenyl)disulfide, and
bis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetra-substituted diphenyl
disulfide such as bis(2,3,5,6-tetra chlorophenyl)disulfide;
penta-substituted diphenyl disulfide such as
bis(2,3,4,5,6-pentachlorophenyl)disulfide and
bis(2,3,4,5,6-pentabromophenyl)disulfide. These diphenyl disulfides or
the derivative thereof can enhance resilience by having some influence on
the state of vulcanization of vulcanized rubber. Among them, diphenyl
disulfide or bis (pentabromophenyl) disulfide is preferably used since
the golf ball having particularly high resilience can be obtained. The
amount of the diphenyl disulfide or the derivative thereof to be blended
is preferably 0.1 part by mass or more, more preferably 0.3 part by mass
or more, and is preferably 5.0 parts by mass or less, more preferably 3.0
parts by mass or less relative to 100 parts by mass of the base rubber.

[0060] The amount of the antioxidant to be blended is preferably 0.1 part
or more and is preferably 1 part or less based on 100 parts of the base
rubber by mass. Further, the amount of the peptizing agent is preferably
0.1 part or more and is preferably 5 parts or less based on 100 parts of
the base rubber by mass.

[0061] The center can be obtained by mixing, kneading the above mentioned
rubber composition and molding the rubber composition in the mold. The
conditions for press-molding the center rubber composition should be
determined depending on the rubber composition. Specifically, the
press-molding is preferably carried out for 10 to 60 minutes at the
temperature of 130° C. to 200° C. under the pressure from
2.9 MPa to 11.8 MPa. Alternatively, the press-molding is preferably
carried out in a two-step heating, for example, for 20 to 40 minutes at
the temperature of 130° C. to 150° C., and continuously for
5 to 15 minutes at the temperature of 160° C. to 180° C.

[0062] The diameter of the center is preferably 34.8 mm or more, more
preferably 35.0 mm or more, and even more preferably 35.2 mm or more, and
is preferably 41.2 mm or less, more preferably 41.0 mm or less, and even
more preferably 40.8 mm or less. If the diameter of the center is 34.8 mm
or more, the intermediate layer or the cover layer does not become
excessively thick, and hence the resilience of the golf ball becomes
good. On the other hand, if the diameter of the center is 41.2 mm or
less, the intermediate layer or the cover does not become excessively
thin, and hence the intermediate layer or the cover functions better.

[0063] When the center has a diameter from 34.8 mm to 41.2 mm, a
compression deformation amount (shrinking deformation amount of the
center along the compression direction) of the center when applying a
load from 98 N as an initial load to 1275 N as a final load is preferably
1.90 mm or more, more preferably 2.00 mm or more, even more preferably
2.10 mm or more, and is preferably 4.00 mm or less, more preferably 3.90
mm or less, even more preferably 3.80 mm or less. If the compression
deformation amount is 1.90 mm or more, the shot feeling becomes good,
while if the compression deformation amount is 4.00 mm or less, the
resilience becomes better.

[0064] The surface hardness of the center is preferably 45 or larger, more
preferably 50 or larger, and even more preferably 55 or larger, and is
preferably 65 or smaller, more preferably 62 or smaller, and even more
preferably 60 or smaller in shore D hardness. If the surface hardness is
45 or more in Shore D hardness, the center does not become excessively
soft and the resilience becomes good. If the surface hardness of the
center is 65 or less in Shore D hardness, the center does not become so
hard and the shot feeling becomes good.

[0065] The intermediate layer is, for example, formed by covering the
center with the golf ball intermediate layer resin composition of the
present invention (hereinafter, sometimes merely referred to as
"intermediate layer composition"). An embodiment for molding the
intermediate layer is not particularly limited, and includes an
embodiment which comprises injection molding the intermediate layer
composition directly onto the center, or an embodiment which comprises
molding the intermediate layer composition into a half hollow-shell,
covering the center with the two half hollow-shells, and subjecting the
center with the two half hollow-shells to the compression-molding at the
temperature of 130° C. to 170° C. for 1 min to 5 mins. The
intermediate layer of the golf ball of the present invention is
preferably formed by injection molding. The intermediate layer can be
produced more easily by injection molding.

[0066] In the case of directly injection molding the intermediate layer
composition onto the center, the intermediate layer composition in the
pellet form may be used for injection molding or the materials such as
the resin components and the pigment may be dry blended, followed by
directly injection molding the blended material. In the present
invention, it is preferable to use the intermediate layer composition in
the pellet form which is obtained by extrusion, because it is possible to
disperse zinc compounds into the resin components homogenously. It is
also preferred to use upper and lower molds for forming the intermediate
layer having a spherical cavity and pimples, wherein a part of the pimple
also serves as a retractable hold pin. When forming the intermediate
layer by injection molding, the hold pin is protruded to hold the center,
and the intermediate layer composition which has been heated and melted
is charged and then cooled to obtain the intermediate layer.

[0067] In the case of molding the intermediate layer using the injection
molding apparatus provided with an injection unit and a mold unit, a
temperature (setting temperature of the injection unit) at the cylinder
(barrel) portion of the injection unit is preferably 200° C. or
more, more preferably 210° C. or more, and is preferably
270° C. or less, more preferably 260° C. or less. If the
temperature of the cylinder (barrel) portion falls within the above
range, the golf ball intermediate layer resin composition can maintain
the fluidity and the neutralization of non-neutralized carboxyl groups
existing in (A) component and (B) component with (C) the zinc compound
proceeds.

[0068] When molding the intermediate layer in a compression molding
method, molding of the half shell can be performed by either compression
molding method or injection molding method, and the compression molding
method is preferred. The compression-molding of the intermediate layer
composition into half shell can be carried out, for example, under a
pressure of 1 MPa or more and 20 MPa or less at a temperature of
-20° C. or more and 70° C. or less relative to the flow
beginning temperature of the intermediate layer composition. By
performing the molding under the above conditions, a half shell having a
uniform thickness can be formed. Examples of a method for molding the
intermediate layer using half shells include compression molding by
covering the center with two half shells. The compression molding of half
shells into the intermediate layer can be carried out, for example, under
a pressure of 0.5 MPa or more and 25 MPa or less at a temperature of
-20° C. or more and 70° C. or less relative to the flow
beginning temperature of the intermediate layer composition. By
performing the molding under the above conditions, the intermediate layer
having a uniform thickness can be formed.

[0069] The molding temperature means the highest temperature where the
temperature at the surface of the concave portion of the lower mold
reaches from closing through opening the molds. Further, the flow
beginning temperature of the intermediate layer composition can be
measured in a pellet form under the following conditions by using a flow
characteristics evaluation apparatus (Flow Tester CFT-500, manufactured
by Shimadzu Corporation). [0070] Measuring conditions: Area size of a
plunger: 1 cm2, Die length: 1 mm, Die diameter: 1 mm, Load: 588.399
N, Start temperature: 30° C., and Temperature increase rate:
3° C./min.

[0071] The thickness of the intermediate layer of the golf ball of the
present invention is preferably 1.5 mm or less, more preferably 1.4 mm or
less, and even more preferably 1.2 mm or less. If the thickness of the
intermediate layer is 1.5 mm or less, the resilience and shot feeling of
the golf ball are improved. The thickness of the intermediate layer is
preferably 0.5 mm or more, more preferably 0.6 mm or more, even more
preferably 0.7 mm or more. If the thickness of the intermediate layer is
0.5 mm or more, it is easier to mold the intermediate layer. Further, the
durability of the resultant golf ball is enhanced.

[0072] In one preferable embodiment that the golf ball of the present
invention has at least two intermediate layers, the golf ball may have an
intermediate layer which is formed from an intermediate layer composition
different from the golf ball intermediate layer resin composition of the
present invention, as long as at least one of the intermediate layer is
formed from the golf ball intermediate layer resin composition of the
present invention. In this case, it is preferred that the outermost layer
of the core is an intermediate layer formed from the golf ball
intermediate layer resin composition of the present invention, and it is
more preferred that all the multi-piece of intermediate layers or
multi-layer of intermediate layers are formed from the golf ball
intermediate layer resin composition of the present invention.

[0073] Examples of the intermediate layer composition which is different
from the golf ball intermediate layer resin composition of the present
invention include, a thermoplastic polyamide elastomer having a trade
name "Pebax (registered trademark) (e.g. "Pebax 2533")" commercially
available from Arkema Inc., a thermoplastic polyester elastomer having a
trade name "Hytrel (registered trademark) (e.g. "Hytrel 3548" and "Hytrel
4047")" commercially available from Du Pont-Toray Co., Ltd., a
thermoplastic polyurethane elastomer having a trade name "Elastollan
(registered trademark) (e.g. "Elastollan XNY97A")" commercially available
from BASF Japan Ltd., a thermoplastic polystyrene elastomer having a
trade name "Rabalon (registered trademark)" commercially available from
Mitsubishi Chemical Corporation, and the like, in addition to the center
rubber composition described above and the ionomer resin. Further, the
intermediate layer composition may contain a specific gravity adjusting
agent such as barium sulfate, tungsten, or the like; an antioxidant; a
pigment; or the like.

[0074] In one preferable embodiment, the diameter of the core of the golf
ball is preferably 39.0 mm or more, more preferably 39.5 mm or more, and
even more preferably 40.8 mm or more. If the diameter of the core is less
than 39.0 mm, the cover becomes excessively thick and hence the
resilience of the golf ball may deteriorate. Further, the diameter of the
core is preferably 42.2 mm or less, more preferably 42.0 mm or less, and
even more preferably 41.8 mm or less. If the diameter of the core is more
than 42.2 mm, the thickness of the cover becomes relatively thin, and
hence a protection effect of the cover is not obtained sufficiently.

[0075] When the core has a diameter from 39.0 mm to 42.2 mm, a compression
deformation amount (shrinking deformation amount of the core along the
compression direction) of the core when applying a load from 98 N as an
initial load to 1275 N as a final load is preferably 1.90 mm or more,
more preferably 2.00 mm or more, even more preferably 2.10 mm or more,
and is preferably 4.00 mm or less, more preferably 3.90 mm or less, even
more preferably 3.80 mm or less. If the compression deformation amount is
less than 1.90 mm, the core becomes too hard, resulting in the poor shot
feeling, while if the compression deformation amount is more than 4.00
mm, the core becomes too soft, resulting in the heavy shot feeling.

[0076] The center hardness of the core is preferably 30 or larger, more
preferably 32 or larger, and even more preferably 35 or larger in Shore D
hardness. If the center hardness is smaller than 30 in Shore D hardness,
the core becomes so soft that the resilience of the golf ball tends to
become lower. The center hardness of the core is preferably 50 or
smaller, more preferably 48 or smaller, and even more preferably 46 or
smaller in Shore D hardness. If the center hardness is more than 50 in
Shore D hardness, the core becomes too hard, resulting in the poor shot
feeling. In the present invention, the center hardness of the core is the
hardness measured with the Shore D type spring hardness tester at the
central point of a cut plane of a core which has been cut into two
halves.

[0077] The surface hardness of the core is preferably 55 or larger, more
preferably 56 or larger, and even more preferably 57 or larger in Shore D
hardness. If the surface hardness is less than 55 in Shore D hardness,
the core becomes so soft and the resilience may be lowered. The surface
hardness of the core is preferably 75 or smaller, more preferably 73 or
smaller, and even more preferably 70 or smaller in shore D hardness. If
the surface hardness is more than 75 in Shore D hardness, the core
becomes so hard and the shot feeling may be lowered.

[0078] In one preferable embodiment, the surface hardness of the core is
made larger than the center hardness of the core. Making the core have
the surface hardness larger than the center hardness provides a golf ball
with a high launch angle and a low spin rate on driver shots. The
hardness difference (surface hardness-center hardness) between the
surface hardness and the center hardness of the core in the golf ball of
the present invention is preferably 4 or larger, more preferably 7 or
larger in Shore D hardness. Further, the hardness difference (surface
hardness-center hardness) between the surface hardness and the center
hardness of the core in the golf ball of the present invention is
preferably 40 or less, more preferably 35 or less. If the hardness
difference is too large, the durability of the golf ball may deteriorate.

[0079] In one preferable embodiment, the cover of the golf ball of the
present invention is formed from a cover composition containing a resin
component. The resin component includes, for example, a thermoplastic
polyamide elastomer having a commercial name of "Pebax (registered
trademark) (e.g. "Pebax 2533")" commercially available from Arkema K. K.;
a thermoplastic polyester elastomer having a commercial name of "Hytrel
(registered trademark) (e.g. "Hytrel 3548", "Hytrel 4047")" commercially
available from Du Pont-Toray Co.,Ltd.; a thermoplastic polystyrene
elastomer having a commercial name of "Rabalon (registered trademark)
(e.g. "Rabalon T3221C")" commercially available from Mitsubishi Chemical
Corporation, in addition to the polyurethane resin and the ionomer resin.
Further, (B) a binary copolymer composed of an olefin and an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms
and/or a ternary copolymer composed of an olefin, an
α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,
and an α,β-unsaturated carboxylic acid ester can be used.
These resin components are used solely or as a mixture of at least two of
them.

[0080] In one preferable embodiment, the cover composition preferably
contains the polyurethane or the ionomer resin, as the resin component.
The content of the polyurethane or the ionomer resin in the resin
component of the cover composition is preferably 50 mass % or more, more
preferably 60 mass % or more, even more preferably 70 mass % or more. In
more preferable embodiment, the cover composition contains the
polyurethane as the resin component. The polyurethane cover improves the
controllability on approach shots.

[0081] In the present invention, in addition to the aforementioned resin
component, the cover composition may further contain a pigment component
such as a white pigment (for example, titanium oxide), a blue pigment, a
red pigment, or the like; a specific gravity adjusting agent such as zinc
oxide, calcium carbonate, barium sulfate, or the like; a dispersant; an
antioxidant; an ultraviolet absorber; a light stabilizer; a fluorescent
material; a fluorescent brightener; or the like, as long as they do not
impair the performance of the cover.

[0082] The amount of the white pigment (for example, titanium oxide), with
respect to 100 parts by mass of the resin component constituting the
cover, is preferably 0.5 part by mass or more, more preferably 1 part by
mass or more, and is preferably 10 parts by mass or less, more preferably
8 parts by mass or less. If the amount of the white pigment is 0.5 parts
by mass or more, it is possible to impart the opacity to the cover. If
the amount of the white pigment is more than 10 parts by mass, the
durability of the cover may deteriorate.

[0083] The cover composition preferably has a slab hardness of 65 or less,
more preferably 60 or less, even more preferably 55 or less in Shore D
hardness. If the cover composition has a slab hardness of 65 or less, the
spin rate on the approach shots with short irons increases. As a result,
the golf ball having a good controllability on the approach shots is
obtained. In order to ensure the spin rate sufficiently for the approach
shots, the cover composition preferably has a slab hardness of 20 or
more, more preferably 25 or more, even more preferably 30 or more in
Shore D hardness.

[0084] An embodiment for molding a cover is not particularly limited, and
includes an embodiment which comprises injection molding the cover
composition directly onto the core, or an embodiment which comprises
molding the cover composition into a hollow-shell, covering the core with
a plurality of the hollow-shells and subjecting the core with a plurality
of the hollow shells to the compression-molding (preferably an embodiment
which comprises molding the cover composition into a half hollow-shell,
covering the core with the two half hollow-shells, and subjecting the
core with the two half hollow-shells to the compression-molding).

[0085] When molding the cover in a compression molding method, molding of
the half shell can be performed by either compression molding method or
injection molding method, and the compression molding method is
preferred. The compression-molding of the cover composition into half
shell can be carried out, for example, under a pressure of 1 MPa or more
and 20 MPa or less at a temperature of -20° C. or more and
70° C. or less relative to the flow beginning temperature of the
cover composition. By performing the molding under the above conditions,
a half shell having a uniform thickness can be formed. Examples of a
method for molding the cover using half shells include compression
molding by covering the core with two half shells. The compression
molding of half shells into the cover can be carried out, for example,
under a pressure of 0.5 MPa or more and 25 MPa or less at a temperature
of -20° C. or more and 70° C. or less relative to the flow
beginning temperature of the cover composition. By performing the molding
under the above conditions, a golf ball cover having a uniform thickness
can be formed.

[0086] In the case of directly injection molding the cover composition
onto the core, it is preferred to use upper and lower molds for forming a
cover having a spherical cavity and pimples, wherein a part of the
pimples also serves as a retractable hold pin. When forming the cover by
injection molding, the hold pin is protruded to hold the core, and the
cover composition which has been heated and melted is charged and then
cooled to obtain a cover. For example, the cover composition heated and
melted at the temperature of 200° C. to 250° C. is charged
into a mold held under the pressure of 9 MPa to 15 MPa for 0.5 to 5
second. After cooling for 10 to 60 seconds, the mold is opened and the
golf ball with the cover molded is taken out from the mold.

[0087] When molding a cover, the concave portions called "dimple" are
usually formed on the surface. The total number of the dimples is
preferably 200 or more and 500 or less. If the total number is less than
200, the dimple effect is hardly obtained. On the other hand, if the
total number exceeds 500, the dimple effect is hardly obtained because
the size of the respective dimples is small. The shape (shape in a plan
view) of dimples includes, for example, without limitation, a circle,
polygonal shapes such as roughly triangular shape, roughly quadrangular
shape, roughly pentagonal shape, and roughly hexagonal shape, another
irregular shape. The shape of the dimples is employed solely or in
combination at least two of them.

[0088] In the present invention, the thickness of the cover of the golf
ball is preferably 2.0 mm or less, more preferably 1.6 mm or less, even
more preferably 1.2 mm or less, most preferably 1.0 mm or less. If the
thickness of the cover is 2.0 mm or less, the resilience and shot feeling
of the obtained golf ball become better. The thickness of the cover is
preferably 0.1 mm or more, more preferably 0.2 mm or more, and even more
preferably 0.3 mm or more. If the thickness of the cover is less than 0.1
mm, it may become difficult to mold the cover. In addition, the
durability and the abrasion resistance of the cover may deteriorate.

[0089] After the cover is molded, the mold is opened and the golf ball
body is taken out from the mold, and as necessary, the golf ball body is
preferably subjected to surface treatments such as deburring, cleaning,
and sandblast. If desired, a paint film or a mark may be formed. The
paint film preferably has a thickness of, but not limited to, 5 μm or
larger, and more preferably 7 μm or larger, and preferably has a
thickness of 25 μm or smaller, and more preferably 18 μm or
smaller. If the thickness is smaller than 5 μm, the paint film is easy
to wear off due to continued use of the golf ball, and if the thickness
is larger than 25 μm, the effect of the dimples is reduced, resulting
in lowering flying performance of the golf ball.

[0090] When the golf ball of the present invention has a diameter in a
range from 40 mm to 45 mm, a compression deformation amount of the golf
ball (an amount of compression of the golf ball in the compression
direction thereof) when applying an initial load of 98 N to a final load
of 1275 N to the golf ball is preferably 2.0 mm or more, more preferably
2.2 mm or more, and is preferably 4.0 mm or less, more preferably 3.5 mm
or less. If the compression deformation amount is 2.0 mm or more, the
golf ball does not become excessively hard, and thus exhibits the good
shot feeling. On the other hand, if the compression deformation amount is
4.0 mm or less, the resilience is enhanced.

EXAMPLES

[0091] Hereinafter, the present invention will be described in detail by
way of example. The present invention is not limited to examples
described below. Various changes and modifications can be made without
departing from the spirit and scope of the present invention.

(1) Hardness of Center and Core (Shore D Hardness)

[0092] A type P1 auto loading durometer manufactured by Kobunshi Keiki
Co., Ltd., provided with a Shore D type spring hardness tester prescribed
in ASTM-D2240 was used to measure the surface hardness and the center
hardness of the center and the core. Shore D hardness measured at the
surfaces of the center and the core are defined as the surface hardness
of the center and the core, respectively. The core was cut into two
hemispheres to obtain a cut plane, and a Shore D hardness measured at the
central point of the cut plane was used as the center hardness of the
core (center).

(2) Slab Hardness (Shore D Hardness)

[0093] Sheets with a thickness of about 2 mm were produced by injection
molding (cylinder temperature: 230° C.) the cover composition and
the intermediate layer composition, and stored at 23° C. for two
weeks. Three or more of these sheets were stacked on one another so as
not to be affected by the measuring substrate on which the sheets were
placed, and the hardness of the stack was measured with a type P1 auto
loading durometer manufactured by Kobunshi Keiki Co., Ltd., provided with
a Shore D type spring hardness tester prescribed in ASTM-D2240.

(3) Compression Deformation Amount (mm)

[0094] A compression deformation amount of the center, core and golf ball
(a shrinking amount of the center, core, and golf ball in the compression
direction thereof), when applying a load from 98 N as an initial load to
1275 N as a final load to the center, core and golf ball, was measured.

(4) Melt Flow Rate (MFR) (g/10 min)

[0095] The MFR was measured using a flow tester (Shimadzu flow tester
CFT-100C manufactured by Shimadzu Corporation) in accordance with JIS
K7210. The measurement was conducted under the conditions of the
measurement temperature 190° C. and the load of 2.16 kg.

(5) Coefficient of Restitution

[0096] A 198.4 g of metal cylindrical object was forced to collide with
each golf ball at a speed of 40 m/sec, and the speeds of the cylindrical
object and the golf ball before and after the collision were measured.
Based on these speeds and the mass of each object, coefficient of
restitution for each golf ball was calculated. The measurement was
conducted by using twelve golf balls for each golf ball, and the average
value was regarded as the coefficient of restitution for the golf ball.
The coefficient of restitution of golf ball No.11 was defined as an index
of 100.0, and the coefficient of restitution of each golf ball was
represented by converting the coefficient of restitution of each golf
ball into this index.

(6) Durability

[0097] A metal-head W#1 driver ("XXIO" manufactured by SRI sports, Shaft
hardness: S, loft angle: 11°) was installed on a swing robot
manufactured by TRUETEMPER CO, and the head speed was set to 45 m/sec.
Each golf ball was stored in a constant temperature reservoir kept at the
temperature of 23° C. for 12 hours. Immediately after taking each
golf ball out of the reservoir, they were repeatedly hit with the driver.
The number of hits required to break the golf ball was counted. This
measurement was conducted by using twelve golf balls for each golf ball.
The number of hits for golf ball No. 11 was defined as an index of 100,
and the durability of each golf ball was represented by converting the
number of hits for each golf ball into this index. A greater index value
indicates that the durability of the golf ball is excellent.

(7) Bending Stiffness (MPa)

[0098] A sheet with a thickness of about 2 mm was produced by an injection
molding (cylinder temperature: 230° C.) from the intermediate
layer composition, and stored at 23° C. for two weeks. The bending
stiffness was measured according to JIS K7106. The measurement was
conducted under the conditions of the temperature 23° C. and
humidity 50RH %.

(8) FT-IR Measurement

[0099] A measuring samples were taken from the pellet of the intermediate
layer composition before injection molding and the intermediate layer
after injection molding. These samples were analyzed with Fourier
transform infrared spectrophotometer (Auto IMAGE FT-IR) available from
PerkinElmer, Inc by a macro ATR method (germanium prism, observation
diameter: about 1 mm). The area A under a peak around 1600 cm-1 and
the area B under a peak around 1700 cm-1 were determined from the
obtained spectrum. FIG. 1 shows a FT-IR spectrum of the intermediate
layer of the golf ball No. 1 (vertical axis: absorbance). Base lines were
drawn for Peak A around 1600 cm-1 and Peak B around 1700 cm-1,
respectively, as shown FIG. 1 and area A, B were calculated. It is noted
that Peak A around 1600 cm-1 may shift depending on kinds of metal
neutralizing the carboxyl group.

[Production of Golf Balls]

(1) Production of Center

[0100] The center rubber compositions having the formulations shown in
Table 1 were kneaded and heat-pressed in upper and lower molds, each
having a hemispherical cavity, at 170° C. for 20 minutes to
prepare spherical centers. The amount of barium sulfate was adjusted
appropriately to make a golf ball have a weight of 45.4 g.

[0101] Blending materials shown in Tables 2 to 6 were mixed with a
twin-screw kneading extruder to prepare the intermediate layer
composition and cover compositions in the pellet form. The extruding
conditions for the intermediate layer composition were a screw diameter
of 45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and the
cylinder temperature of 140 to 200° C. The extruding conditions
for the cover composition were a screw diameter of 45 mm, a screw
rotational speed of 200 rpm, and screw L/D=35, and the mixtures were
heated to 160 to 230° C. at the die position of the extruder.

[0102] The intermediate layer compositions obtained above were
injection-molded onto the spherical centers to form the intermediate
layers covering the centers and prepare spherical cores. Subsequently,
golf balls were produced by injection-molding the cover composition onto
the spherical cores to form the cover. Upper and lower molds for the
intermediate layer and the cover have a spherical cavity with pimples, a
part of pimples serves as a hold pin which is retractable.

[0103] Upper and lower molds for the intermediate layer have a spherical
cavity with pimples, a part of pimples serves as a hold pin which is
retractable. When molding the intermediate layer, the hold pins were
protruded to hold the center after the center was put in. The
intermediate layer composition was heated to 200° C. to
260° C. at the cylinder portion of the injection unit and charged
into the mold held under a pressure of 15 MPa, and cooled for 30 seconds.
Then, the mold was opened, and the cores were taken out from the mold.

[0104] When molding the cover, the hold pins were protruded to hold the
core after the core was put in, the cover composition heated to
260° C. was charged into the mold under a pressure of 80 tons
within 0.3 seconds, and cooled for 30 seconds. Then, the mold was opened,
and the golf ball bodies were taken out from the mold. The surface of the
obtained golf ball bodies were treated with sandblast, marked, and
painted with a clear paint. The paint was dried in an oven at 40°
C. to form a paint film, and golf balls having a diameter of 42.8 mm and
a mass of 45.4 g were obtained. The results of evaluations with respect
to the compression deformation amount and resilience of the golf balls
were also shown in tables 3 to 6.

[0118] The golf ball intermediate layer resin compositions comprising, (A)
an ionomer resin consisting of a metal ion-neutralized product of a
binary copolymer composed of an olefin and an α,β-unsaturated
carboxylic acid having 3 to 8 carbon atoms; (B) a binary copolymer
composed of an olefin and an α,β-unsaturated carboxylic acid
having 3 to 8 carbon atoms, and (C) a zinc compound, wherein a content
ratio ((A)/(B)) of (A) component to (B) component ranges from 50/50 to
80/20 in a mass ratio and a content of (C) component ranges from 0.1 part
to 20 parts with respect to 100 parts by mass of a sum of (A) component
and (B) component, and the golf ball intermediate layer resin composition
has a melt flow rate (190° C., 2.16 kg) of 15 g/10 min or more
were excellent in fluidity, and enabled to mold a thin cover. Further,
P2/P1 were 1.5 or more in every golf ball intermediate layer resin
composition of the present invention, thus it is confirmed that the
neutralization was conducted with (C) the zinc compound. Golf balls No. 1
to No. 10, and No. 14 using these golf ball intermediate layer resin
compositions were excellent in the durability and flight distance,
irrespective of having an intermediate layer with a high hardness.

[0119] Golf ball No. 12 is the case that the metal salt of a fatty acid
was used to improve the fluidity. Since the mechanical properties of the
intermediate layer was lowered, the durability of the golf ball was
lowered. Because of too low fluidity, it was impossible to mold Golf ball
No. 13. Golf ball No. 15 is the case that (C) the zinc compound was not
contained. The durability was lowered. In Golf ball No.16, since the melt
flow rate was as low as 13 g/10 min, it was necessary to increase the
molding temperature to 300° C. Therefore, the resin component
constituting the intermediate layer composition thermally decomposed and
the durability was lowered. Golf ball No. 17 was the case that the
content of (A) component was too high. It was impossible to mold the golf
ball. Golf ball No. 18 was the case the content of (A) component was too
low. The resilience and the durability was lowered.

[0120] The present invention is suitable for the golf ball having an
ionomer intermediate layer. This application is based on Japanese Patent
application No. 2010-147815 filed on Jun. 29, 2010, the contents of which
are hereby incorporated by reference.